Crystal structure of human apolipoprotein A-I: insights into its protective effect against cardiovascular diseases

Abstract

Despite three decades of extensive studies on human apolipoprotein A-I (apoA-I), the major protein component in high-density lipoproteins, the molecular basis for its antiatherogenic function is elusive, in part because of lack of a structure of the full-length protein. We describe here the crystal structure of lipid-free apoA-I at 2.4 A. The structure shows that apoA-I is comprised of an N-terminal four-helix bundle and two C-terminal helices. The N-terminal domain plays a prominent role in maintaining its lipid-free conformation, indicating that mutants with truncations in this region form inadequate models for explaining functional properties of apoA-I. A model for transformation of the lipid-free conformation to the high-density lipoprotein-bound form follows from an analysis of solvent-accessible hydrophobic patches on the surface of the structure and their proximity to the hydrophobic core of the four-helix bundle. The crystal structure of human apoA-I displays a hitherto-unobserved array of positively and negatively charged areas on the surface. Positioning of the charged surface patches relative to hydrophobic regions near the C terminus of the protein offers insights into its interaction with cell-surface components of the reverse cholesterol transport pathway and antiatherogenic properties of this protein. This structure provides a much-needed structural template for exploration of molecular mechanisms by which human apoA-I ameliorates atherosclerosis and inflammatory diseases.

Fig. 1.

Overall stereoview of the structure. The six helices in the structure are rendered as C^α worms, colored blue (A), pink (B), yellow (C), lavender (D), cyan (E), and red (F) and labeled. Loops are colored gold. Hydrophobic residues are shown as green sticks.

Fig. 2.

Comparison with other apolipoproteins. (A) Stereoview of superimposition of the C^α worm of lipid-free apoA-I over that of the N-terminal domain of apoE. ApoA-I is colored gold, and apoE is blue. (B) Full-length apoA-I, shown within the elliptical loop formed by the Δ1–43 structure. Both structures are oriented to place their two largest dimensions in the figure plane and mutually aligned. Helices A–F in the full-length structure are colored as in Fig. 1. Helices in Δ1–43 are colored to match the sequence limits of the helices in the full-length protein.

Fig. 3.

Model for lipid-assisted conversion. Helices are represented as cylinders and colored as in Fig. 1. Hydrophobic residues are depicted as green sticks. The C-terminal domain is shown as a C^α worm in an arbitrary orientation and position as a single, long helix. (A) Initial lipid-free conformation of apoA-I. Residues contributing to the hydrophobic patch at the N terminus of helix 1 are shown. (B and C) The two geometrically possible open conformations, formed through lipid binding to the bundle. The energetically more likely conformation (C) is indicated by a solid arrow. Hydrophobic side chains that contribute to the four-helix bundle interface are exposed. (D) Rearrangement of a fraction of the open form into a stable helix–hairpin intermediate. Residues forming the new hydrophobic stabilization core are illustrated. (E) Putative conformation in the HDL-bound form, represented by the Δ1–43 structure. Hydrophobic residues that could potentially form the interaction interface with lipid in HDL are shown.

Fig. 4.

Surface hydrophobic and electrostatic properties. (A) Solvent-accessible surface of apoA-I (gray) with helix 1 residues omitted. Hydrophobic patches are colored green with side chains of leucine quartet colored purple superimposed on the patch at the base of helix 1. C^α worm of helix 1 in light purple and internal hydrophobic residues in cyan are also shown. (B) Positive (blue) and negative (red) electrostatic potential mapped to the molecular surface of apoA-I, oriented to show the largest negative patch. Contours are displayed at ±10e. Positions of side-chain carboxyls are labeled.

Fig. 5.

Functional importance of some residues. Stereoview of C^α worm of lipid-free apoA-I, colored light purple, with N-terminal helix, LCAT interaction region, and residues 98–108 highlighted in violet, red, and blue, respectively. Side chains of some hydrophobic residues, which interact with LCAT in the lipid-bound form of apoA-I but are part of the hydrophobic core in this structure, are shown in green. Tyr-192, the target for myeloperoxidase, is shown in gold.